Page:Popular Science Monthly Volume 21.djvu/439

Rh this singular circumstance, a special form of parasitism, which he calls reciprocal parasitism. It may, perhaps, be illustrated by the case of the leaf of the maple, which is frequently attacked at the end of the summer by an erysiphus, that occupies the lower surface and fruits there. In the fall the leaves change color and drop off, but the spots occupied by the erysiphus remain green, and so continue, a spot distinct from all the rest of the leaf, for weeks. A similar reciprocation may take place between the algoid and fungoid parts of the lichen.

A Mechanical Rock-Excavator.—By the aid of perforators, worked by compressed air, it has been possible to complete in a few years works of subterranean excavation like the tunnels of Mont Cenis and Mont St. Gothard, which would formerly have occupied several generations of miners. Hand-drilling can be dispensed with when such perforators are used, for the perforator itself makes the hole destined to receive the powder by which the rock is blown up, and the miner has nothing to do but to pick up the pieces. A full solution of the mechanical operation of boring is thus obtained, and a self-acting ventilation of the shafts and the removal of all noxious vapors are secured at the same time. Till recently it was not practicable to apply the perforators in coal-mines charged with explosive gases, for such mines had to be worked without using powder. Two French engineers, Mil. Dubois and Francois, have resolved this difficulty by the construction of an apparatus which they call a bosseyeuse, or, as we might call it in English, a pioneer-drill. Its principal feature is a kind of needle-wedge which is made under the action of the perforator to enter the hole made by the drill and break the rocks up by the force of pressure, without throwing out the fragments as powder does. This apparatus has been used in several gas-infected mines in the Belgian coal-basin, with a considerable consequent diminution in the frequency of



explosions. The powder, if that is used, is introduced when the hole excavated by the perforator has reached a depth of about twenty-seven inches; or, if the mine contains explosive gases, the needle-wedge is substituted for powder. This apparatus, which is represented in the figures (1 to 4), consists of two bars of iron, shaped like segments of a circle, and touching by their ends, the union of which constitutes a cylinder hollowed along its axis with a conical hole. The section of the two segments is of the same size as the drill-hole, so that they fill it completely when inserted in it. As soon as the drill-hole has been hollowed to a sufficient depth, these two bars are